On a new theory of nervous action as regards the transmission of sensation along the nerves

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A NEW THEORY OF NERVOUS ACTION

AS REGARDS THE TRANSMISSION OF SENSATION ALONG THE NERVES.

BY ROBERT M‘ DONNELL, M. D., F. R. S.

[Abstract]

[Read before the Royal Irish Academy, May 23, 1870.]

1 large number of facts have of late years been observed, tending to
how that what has hitherto been regarded as the sense of touch is ca-
pable of being resolved into a number of comparatively elementary sen-
sations, as those of temperature, contact, tickling, pain, &c.

Many cases have likewise been observed in which some of these
: ensations are felt, while others cease to be perceived by the patient.
Thus the individual may feel perfectly the contact of the hand, when
ightly rubbed over the surface, yet not be able to distinguish heat from
old, or vice versd.

Analogous phenomena are observed with regard to the other senses,
:s in cases of colour blindness, absence of ear, or inability to hear par-
icular notes, &c.

In explanation of these and other kindred phenomena, it has been
:upposed that there exist in every nerve groups of distinct conductors,
iach adapted to convey along it distinct nervous impressions.

This hypothesis is indeed that which is at the present time adopted
py.phyeiologists, and it numbers among its supporters the most distin-
guished philosophers.

Dr. Brown-Sequard conceives “that he has ascertained that, beside
he four distinct kinds of nerve fibres of the higher senses, there are at
(‘■east eleven kinds of nerve fibres in the spinal cord, and in the cranial,

I pinal, and sympathetic nerves.’’

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He enumerates these eleven kinds as follows

1 st. Conductors of impressions of touch.

2nd.

3rd.

4th.

5th. _ „

6th. Incito-motor conductors.

7th. Incito-nutritive and secretory conductors.
8th. Voluntary motor conductors.

9th. Involuntary motor conductors.

10th. Vaso-motor conductors.

11th. Nutritive and secretory conductors.

of tickling,
of pain.

of temperature,
of muscular contraction.

“ I hardly need say,” he adds, “that the number of functionally dis-
tinct nerve fibres is probably much greater than is shown in this
table.”

As regards the physiology of sensations of colour, a theory so closely
analogous as indeed to be identical with reference to the sense of vision
was put forward by Thomas Young, at the commencement of this cen-
tury. He supposed three sorts of conductors to exist in the optic nerve,
each specially charged with the function of conducting a different colour,
red, green, and violet. The mixture of these three colours in different
proportions gave rise to all the other colours of the spectrum.

This hypothesis of Young has, with some modifications as to the
colours, found a zealous advocate in the distinguished Professor Helm-
holtz.

It is not necessary for my purpose to enumerate the various theories
which have been advanced in explanation of the various phenomena to
which 1 have just alluded. Suffice it to say that I have long felt that
the ingenious idea of distinct conductor's did not exactly meet the case.
So long ago as in 1861, in a critique on Dr. Brown- Sequard’s work in
which his theory was first put forward, I expressed the opinion that we
could hardly accept the idea “ that the nerve fibres employed in the trans-
mission of sensitive impressions of touch , tickling, pain, &c., are as dis-
tinct one from the other as they all are, from the nerve fibres employed
in the transmissions of the orders of the will to the muscles.”

The theory which I venture to propose, and which I put forward
with diffidence when I consider that another has been advocated by
such able physiologists as Helmholtz and Brown-Sequard, is simply an
application of the theory of wave propagation to the passage of various
sensations along nerve conductors.

I conceive that the various peripheral expansions of sensitive nerves
take up undulations or vibrations, and convert them into waves capable
of being propagated along nervous tissue (neuricity, as it has been
named). Thus, the same nerve tubule may be able to transmit along
it vibrations differing in character, and hence, giving rise to different
sensations ; and, consequently, the same nerve tubule may, in its normal

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condition, transmit the wave which produces the idea of simple contact,
or that which produces the idea of heat — or, again, the same nerve
tubules in the optic nerve which propagate the undulations of red may
also propagate, in normal vision, those which excite the idea of yellow
or blue, and so for other senses.

I advocate this undulatory theory of sensation in preference to the
theory of distinct conductors —

lstly. because it is simple.

2ndly. Because it is strongly supported by analogy, when compared
with wave propagations in other departments of science.

3rdly. Because it appears to be in harmony with a large number
of recognized physiological facts, which seem inexplicable
upon the theory of distinct conductors.

It would be obviously impossible, within the limits of one com-
munication, to discuss such a theory in its application to the various
senses. I wish merely to bring before the Academy, at present, a
general statement of the grounds upon which this hypothesis rests ; and
I shall hope, hereafter, in several communications, to elucidate its
applicability to the transmission of the sensations peculiar to each special
sense.

1st. When compared with the theory of distinct conductors, the
undulatory theory is obviously simpler as regards anatomical detail.
Anatomy has not given any evidence that with an ordinary compound
nerve there exist different kinds of conductors — to the highest powers of
the microscope all such tubules are identical in appearance. A ay more,
we now know that nerves may be so spliced (if I may use the expres-
sion) onto one another, that sensitive nerves may be made continuous
with those wliich convey the commands of the will to muscles.

As regards the analogy between this theory of nerve action and
the wave theory of light, I do not pretend to say that it holds in every
respect : there are obvious points of difference. If we infer that light
and heat do not consist of particles emitted by a hot body, our natural
alternative is to suppose that they are undulations of a medium per-
vading space. This hypothesis furnishes by far the best explanation
of many very curious phenomena in light and heat, and is now generally
received. This medium, which we suppose to pervade space likewise,
with more or less freedom, pervades transparent and diatliermanous
bodies ; but nerve tissue being neither transparent nor diathermanous,
it is not to be conceived that the undulations of this medium are trans-
mitted along nerve tissue, as if through glass or rock salt : on the con-
trary, the vibrations of light and heat are transferred from the medium
in question to the axis cylinder of the nerve tubule in a form capable
of being propagated to the sensorium.

As I conceive, the analogy lies chiefly in this : — as we know, various
solid and liquid bodies exercise a selective absorption both for heat and
light, in virtue of whiclpcertain rays are set apart to be stopped, while
• certain others are allowed to proceed ; after an analogous fashion,
certain nerves exercise a so-called selective power, permitting certain

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undulations to proceed, while those of a different wave length are in-
tercepted. Most substances, including those that are transparent for
light, are generally opaque for dark heat of great wave length and
small refrangibility. So we have no reason to think that heat can
excite in the retina undulations capable of being propagated by the
optic nerve to the sensorium, although light certainly does so.

Instead of supposing, like Dr. Brown-Sequard, that there exist a
great number of distinct conductors, I should suppose that there are a
great number of distinct sensations propagated along the nerve tubules,
in undulations of different wave lengths.

As the rays of the heat, light, and actinic spectra differ in refran-
gibility, so do the undulations produced by heat, cold, pain, tickling,
or the unfelt sensations (if I may use the phrase), — which last cor-
respond to the invisible and cold actinic rays.

As in the economy of nature tire actinic rays play a part of vast
importance, so these vibrations, which play along our nerves, without
our knowing it,, are all important in the animal economy.

The unfelt tickling, which keeps the heart in regular and ceaseless
action during life, is not less important to man than that part of the
sunbeam which we cannot see, nor yet feel the warmth of, is in the
economy of nature.

Many phenomena such as those connected with seeing comple-
mentary colours, when a white surface is gazed at, after the eyes have
been fixed upon a blue, red, or yellow disc ; the phenomena, connected
with peculiar colour, seen after the administration of santoniue ; the
effects of lead poisoning upon sensation, &c., &c., are more easily ex-
plicable upon the undulatory than upon any other hypothesis of sen-
satiou.

The author concluded by referring to the well-known experiments of
Professor Tyndall, showing the power of absorption of vapours and
scents, of which minute quantities are introduced iuto dry air filling
a glass tube. In these experiments a physical change of almost incon-
ceivable subtlety is followed by the interception of waves of radiant
heat. So with a nerve tubule — a minute quantity, suppose, of santo-
nine, entering into the axis cylinder of the optic nerve tubules (as the
scent in the air filling the glass tube), intercepts some light waves of a
certain refrangibility ; and the result is, that all objects looked upon
have their natural colour, minus the intercepted undulations. This
analogy serves to explain the general bearing of this hypothesis.

THE END.